You know what? Nothing needs to be pretty. Why stop at work tools? A home is there to let you cook, sleep and live efficiently. No need to be pretty. A city is there to allow you to go from A to B without any fuzz and to provide the essential services. Pretty cities are annoying.

I have several accounts and thousands of mails. But I can't see how an efficient tool can't be pretty and how a pretty tool can't be efficient.

I agree with the pros you find to the UI of Thunderbird and that's why I use it. But non-prettiness is not a feature. Prettiness is. For most people, it will be more enjoyable, more so if they spend hours each day using the tool, which is more likely in a work environment.

If it's more enjoyable, more people will use it instead of all these non-free pieces of software you listed (and which I will not use as a consequence), which in turn might bring more funding, which might allow the Thunderbird team to make it even more reliable.

Life is there to be enjoyed and this includes work. I also use thunderbird for my personal email account so it's not just work for me, like many people out there.

Why are we even arguing for non-prettiness? This is madness.

Again, the revamping we are talking about is being done for maintainability reasons, which is what you want for your tool to remain efficient, stable and reliable.

I understand the concerns, UI rewrites are often upsetting, but the amount of resistance to change here is quite impressive.

I don't see the point of not wanting improvements. Of course I won't be happy if Thunderbird becomes less reliable or less efficient but we are not there yet.

I trust the Thunderbird team to do the right things. They have not failed me for almost 20 years. I can't use anything else because I'm too used to its UI, the keyboard shortcuts, everything. The first versions after the rewrite might not work very well and have bugs but we can always wait a bit before upgrading.